A turbojet comprises, from upstream to downstream in the direction of the gas flow, a fan, one or more compressor stages, a combustion chamber, one or more turbine stages and a gas exhaust nozzle. These are the gases of the primary stream of the turbojet, that is to say the gas stream at the outlet from the turbine stages and originating from the combustion chamber that escape via the gas exhaust nozzle.
The nozzle usually comprises an outer nozzle casing that delimits the outer envelope of the gas jet of the exhaust stream, as well as an inner nozzle casing that delimits the inner envelope of the gas jet of the exhaust stream to help with its flow. The inner nozzle casing forms a central body of the nozzle and has the shape of a wall having a surface of revolution, that may be cylindrical, conical or more usually of an aerodynamic shape adapted to the stream, that is axisymmetric about the axis of the turbojet. Those skilled in the art usually call this central nozzle body the “plug”. It will hereafter be called the central nozzle body or central body.
Note that the wall of the central nozzle body may also not be axisymmetric for reasons of reducing the noise of the gas jet or reducing the infrared signature; the central body may, in this case, have a wavy, rectangular or elliptical cross section for example. The present invention applies particularly well to a central body with an axisymmetric wall—as is usually the case in civil applications—but also applies to a central body that is not axisymmetric.
In certain turbojets, the central body fulfills another function that is to guide the turbojet vapor relief stream. Specifically, on certain turbojets, at the downstream end of its central shaft, a vapor relief orifice is provided through which various fluids such as oil vapor, certain cooling gases, etc. escape. This is usually called the deaerator. In this case, either a tube to guide the vapor relief stream extends within the central body to its end, for channeled guidance of the vapor relief stream, or no pipe is provided, the central body providing, via its inner surface, the guidance of the vapor relief stream. Vapor relief is usually carried out by aspiration, the pressure within the pipe or the central body being less than the pressure in the turbojet enclosure.
A constant problem for the manufacturers of engines is noise reduction, particularly for the comfort of the passengers and the inhabitants of the zones overflown by the aircraft. It is therefore advisable to attenuate the noise, particularly the noise in the nozzle, that consists of the noise of the combustion chamber, or combustion noise, that is at low frequencies, and the noise of the high and low pressure turbines, or turbine noise, that is at higher frequencies. With conventional passive acoustic coatings, that is to say devices whose geometry is fixed, the low frequency combustion noise cannot be attenuated with the outer nozzle casing, because the volume at this location is insufficient. It is therefore the higher frequency turbine noise that may be treated on the outer nozzle casing, for example with the aid of a thin layer of a material formed into honeycombs. The problem posed therefore is that of attenuating the low frequency combustion noise.